The present invention relates to a system for controlling the braking of a vehicle.
In conventional braking systems, a pressure is applied to each brake unit; the braking torque generated from the pressure acting on a piston (or pistons) forces some friction material to act on a rotating element (usually a drum or a disk). The same pressure is normally applied to both wheels of an axle. Frequently to prevent vehicle instability due to locking of the rear wheels before lockup of the front wheels, a proportioning valve (or valves) modulates the rear wheel brake pressure. Sometimes an adjustable proportioning valve, sensitive to vehicle spring deflection, is used which attempts to maintain proper brake balance. The adjustment rebalances the brakes (front to rear) as a result of changes in vehicle loading.
The usual (conventional) approach has some shortcomings. The coefficient of friction between the brake pads/shoes and the rotating element is not uniform. In high-volume production, the coefficient of friction may vary significantly batch to batch. Allowances are made for this variation, to insure that the rear wheels do not lock prematurely, causing vehicle instability. This "safety margin" results in less than best brake balance. Furthermore, the usual brake balancing schemes assume a nominal friction coefficient of friction of the brake pads and linings and also assume that the reaction torque at the tire/road interface is consistently related to brake torque. This is not always true since if the tire size (rolling radius) changes appreciably, the lever arm, through which the torque acts, will also change.
These shortcomings result in compromises in brake effectiveness, and can cause uneven wear of tires, brake linings, etc. A cost penalty, as well as some degradation in reliability, results from the addition of a load-sensing or a deceleration sensing proportioning valve to adjust the rear braking pressure as a function of the front braking pressure.
Although not a functional element, the subjective reaction of drivers to the perception of force required versus pedal travel, is limited by the need to provide enough fluid displacement to place the brake shoes/pads in contact with the drum/rotor. Pedal travel is dependent on the compliance of a brake and is also affected by any air entrained in the brake hydraulic fluid. The above factors determine the stroke of the master cylinder and therefore the stroke brake pedal. Geometry of the pedal/master cylinder combination can be varied within limits, but for ease of use, excessive travel of the pedal cannot be accommodated. Further, to develop the pedal force required to produce a pressure adequate to stop the vehicle under worst-case conditions, there are other constraints based on physical limitations of the operator.
The present invention is directed to a substantially improved method of regulating the braking effort at the individual wheels, which obviates most or all of the deficiencies noted above.
Specifically, the invention describes a method of regulating the rate of change of velocity (deceleration) of each wheel, based on a deceleration command generated by the operator. The invention envisions an electrically controlled braking system, where the input is an electrical signal derived by any of several sensing processes. The input signal is then examined by a microcontroller, and the appropriate braking activity at each wheel produced.
It is an object of the present invention to provide a braking system in which an allowance is made for differing wheel speeds when negotiating a bend or a corner. A further object of the present invention is to provide a braking system which accommodates the use of different sized tires such as resulting from the use of a temporary spare tire, the use of a severely deflated tire, or the replacement of a previously worn tire with a new one. A further object of the present invention is to provide for antilock braking control with no additional hardware and as such, to provide a cost effective braking system.
Accordingly, the invention comprises: a method of controlling the braking behavior of various wheels of a vehicle during a normal braking mode of operation and during an antilock braking mode of operation. The method uses a control unit or microcontroller which calculates various parameters over a known time or sampling increment. The method including the steps of:
1.1 obtaining a value of the actual rotational velocity of each wheel (W.sub.1,W.sub.2);
1.2 generating a first signal indicative of braking effort P(in) or desired vehicle deceleration;
1.3 generating a per wheel velocity command signal the slope or deceleration of which is proportional to the desired vehicle deceleration p(in) and a multiplicative scale factor (ABSG.sub.i +G.sub.i) for adjusting such deceleration during the normal braking mode and antilock braking mode,
1.4 generating a per wheel error signal Ei as a difference between the wheel velocity command and actual rotational velocity of each wheel,
1.5 operating upon the error signal Ei to generate a brake activity command signal B.sub.i,
1.6 regulating brake force in response to the brake activity command signal B.sub.i.
Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.